Fracturing entails pumping large volumes of high pressure water and chemicals into a formation to initiate and propagate fractures emanating from a borehole. The proppants that are used are intended to lodge in the fractures to hold them open to facilitate subsequent production from that borehole or adjacent boreholes to the surface. While the volumes of the pumped fluid and the pressure at which such fluid is delivered can be measured, it is at best an indirect approximation of the fracture network that has been created in part because the width of the fracture is unknown and variable so that knowing the fracture volume does not allow one to estimate its area.
To gain further knowledge of the extent of the fracture network acoustic techniques have been suggested where the signal is generated from implosion of voids or explosions in a material delivered with the frac fluid. Some relevant background for such acoustic techniques is: US Publication 2009/0125240 USING MICROSEISMIC DATA TO CHARACTERIZE HYDRAULIC FRACTURES, SCHLUMBERGER; US Publication 2011/0188347 VOLUME IMAGING FOR HYDRAULIC FRACTURE CHARACTERIZATION, SCHLUMBERGER; U.S. Pat. No. 6,488,116 Acoustic receiver, Exxon; U.S. Pat. No. 5,963,508 System and method for determining earth fracture propagation,
Atlantic Richfield Company; U.S. Pat. No. 5,917,160 Single well system for mapping sources of acoustic energy, Exxon; U.S. Pat. No. 5,574,218 Determining the length and azimuth of fractures in earth formations, Atlantic Richfield Company; U.S. Pat. No. 5,010,527 Method for determining the depth of a hydraulic fracture zone in the earth, Gas Research Institute; U.S. Pat. No. 4,744,245 Acoustic measurements in rock formations for determining fracture orientation, Atlantic Richfield Company; U.S. Pat. No. 6,840,318 Method for Treating a Subterranean Formation (Enteric Coatings for Treatments), Schlumberger; 1993 Kumar—Bubble Cavitation Power Spectrum FIG. 18; 2000 Pulli & Harben—Imploding (Macroscopic) Glass Spheres FIG. 6 Freq Distribution to 5 Hz Plasma (sparker) sound source mostly 20-200 Hz; Jasco Pocket Book 3rd ed. Underwater Reference & Freq v. Source Air gun Freq Spectrum FIG. 8; 1997 Deanne—Sound generation by bubbles and waves in ocean FIG. 17a Spectral Density.pdf 1993 Cook—Spark Generated Bubbles Power Spectrum p 127; 1974 Underwater Low Frequency Sound Sources Air Gun FIG. 30p 75 Acoustic Frequency Distribution & p 79 Low Freq Cutoffs of Dif Sources.
The present invention addresses a different technique for signal generation that results in a measurable signal, preferably electromagnetic, that is triggered with preferably a pressure pulse using explosive material or other means of generated pressure energy to create the desired signal. In one embodiment the pressure pulse acts on piezoelectric materials to cause an array of measured signals. These and other aspects of the present invention will be more readily apparent from the detailed description and the associated drawing of the preferred embodiment while understanding that the full scope of the invention is to be determined from the appended claims.